Psychological tests are the bread and butter—the currency—of psychological science. Research in psychology depends upon the measurement of psychological traits, which can only be accomplished with psychological tests. Nonetheless, psychological traits are inherently difficult to assess. They are not concrete objects that are obviously measured, like the number of green peas or the height of a giraffe. They are abstract and intangible traits like love or happiness or self-esteem that can neither be seen, touched, nor counted and may be interpreted differently by different people. Therefore a critical part of psychological research involves the construction of tests that can measure psychological traits in a systematic and reliable way.